1. Field of the Invention
The invention relates to a method for welding. More specifically, the invention is concerned with a method for welding that provides for increased control of the weld composition and an increased weld deposition rate.
2. Description of the Related Art
Welding is often not an ideal method of manufacture or repair. Achieving weld metal deposits to satisfy composition, material properties and production requirements cannot always be accomplished with readily available or off-the-shelf welding consumables. Special melts or lots of bare wire, rod, or fluxes can be obtained, but not without any combination of impediments, such as observing minimum quantities, extended lead times, and premium prices. Due to manufacturing restrictions certain chemical composition or mechanical requirements cannot even be achieved with special consumables. Desires to increase production deposition rates are commonly thwarted by complex component geometry and the limitations of the available shop equipment and power, including the chosen welding process.
Two particular welding methods show opportunity for improvement. The submerged arc (SAW) and electroslag (ESW) welding processes have been used for several decades to deposit high quality corrosion resistant or hardfacing deposits using the strip clad technique in a variety of industries. In the power industry, SAW and ESW have seen extensive use in the cladding of reactor pressure vessels, piping, and large bowl, or coal mill rolls where corrosion or wear present problems. ESW and SAW are also used in surfacing techniques where the objective is to deposit additional material to counteract or anticipate the effects of erosion or abrasion. The difference between surfacing and cladding techniques resides more in the desired material composition and function than the method used to apply the material.
These processes and other traditional arc-welding processes suffer from limitations associated with the ability to control (or vary) deposit composition. Thus, the composition of the weld is approximately constant over the surface of the product, even though different surface material properties and, therefore, a different weld deposit composition are desired. Composition has also traditionally been restricted to commercially available wrought wire and strip alloys, and production fluxes. The manufacture of tailored consumable compositions is often expensive since the manufacturer must melt an entire heat (or load) of material to use the processing equipment efficiently. Such heats can range in size from 2000-20,000 lbs. Since a typical consumable costs approximately $8.00 per pound one can easily see that it becomes tremendously expensive to develop “tailored compositions.”
SAW, ESW, and traditional arc-welding methods are also limited in their abilities to vary the quantity of the weld deposited. Arbitrary variation in the weld deposit is difficult because typical consumable electrodes present arc maintenance problems (or the ESW equivalent) related to the power supplied, feed rate, feed angles, electrode extension, and arc length. The feed angle is that angle between the consumable electrode and the welded piece. The arc length is the distance between the two. These issues combine to make variation in the feed rate a non-trivial change in the welding process.
Should two such electrodes be used to make one deposit, the arcs of such electrodes also become unstable if fed at different rates. The additional angles and gaps between the second consumable electrode and both the first consumable electrode and the welded piece must be controlled. This further limits the potential weld composition variation and makes any variation at all more difficult. The electrode material is finally limited to materials that conduct electricity in the manner necessary to establish an arc and melt at a desired rate.
The combination of two consumable electrodes also presents the further problem of “arc blow.” Arc blow is an undesired phenomena in which a weld deposit splatters instead of flowing to the intended location. Arc blow can occur because flowing current creates a magnetic field. With two electrodes the magnetic field from one can repulse the other resulting in force on the flowing melted weld metal to the point it splatters and potentially extinguishes the arc.
In general, where weld deposit consumables are current carrying electrodes, any variation in the feed rate of a single electrode affects the power to the weld puddle and, thus, the overall weld process including heat input, weld puddle geometry, weld composition, and deposition rate. There is, therefore, a need in the industry for a better welding method to easily and economically vary composition, improve weld deposition rates, provide better control of heat input, and weld puddle geometry.